We're actually working on this, first with a rewrite of our item/block behavior system to be modular and moddable using lamdbas.
Then we will be adding a way for plugins to provide new textures in resource packs.
Once that's all done, we will create a client Forge mod to allow for plugins to add new blocks rather than reuse existing ones.
We also have a pretty nice modular system for AI tasks, but it needs more stress testing with a pathfinding implementation before we start finalizing the API for it.